1. Field of Use
This invention pertains to high powered electronic alerting and warning apparatuses for producing high volume siren tones. More particularly, the present invention pertains to a high efficiency electronic siren having multiple phase and amplitude matched horns fed by a common driver.
2. Background of the Prior Art
Electronic alerting and warning apparatuses comprise one or more loudspeakers or drivers, each having a diaphragm that is usually enclosed in a rigid housing which cooperates with the diaphragm to define in front of it an acoustical impedance chamber with a restricted output port. The housing also defines a closed chamber behind the diaphragm that contains a permanent magnet secured to the housing and the coil secured to the diaphragm. When the coil is energized with an alternating current, it cooperates with the magnet to cause vibration of the diaphragm that imparts acoustical energy to the air in front of the diaphragm. If the diaphragm of such a driver were to confront free air, very little load would be imposed upon it, and the excursions of the diaphragm, controlled only by its own stiffness and mass, could attain such amplitude that the diaphragm would be damaged. Instead, the acoustical impedance chamber in front of the diaphragm, with its restricted output port, imposes a load upon the diaphragm that limits its excursions to safe amplitude values. The impedance chamber also improves transfer of energy from the diaphragm to the free air. In conventional apparatuses, such energy transfer is further improved by connecting the driver to an appropriately designed horn that has a narrow throat portion and diverges to a flaring mouth. Typically, each horn has its own driver or set of drivers. The output port of the driver impedance chamber opens coaxially into the throat portion of the horn, which provides a restricted channel that further loads the diaphragm. The divergent portion of the horn is designed for projection of the sound output in a desired beam width.
To achieve the necessary high tonal volume desired of alerting and warning apparatuses, the prior art has addressed the problem by simply adding more horns, each horn having its own driver or set of drivers. Because the properties of air are such that there tends to be an upper limit to the load that can be imposed upon a driver diaphragm by acoustical impedance means, and correspondingly there tends to be an upper limit to the electrical power that can be safely applied to the driver, the necessity for increased volume has been addressed by increasing the number of horns and drivers. Incorporating a number of electronic drivers into outdoor alerting and warning apparatuses is disclosed in U.S. Pat. No. 4,796,009 which discloses a warning device having numerous electronic drivers attached to a resonance chamber having one or two coaxial outlet ports each opening into the throat of a horn. This type of approach is typical of the prior art and simply involves throwing more power at the problem rather than increasing efficiency. Increasing power generated by a warning apparatus has also been addressed by improved energizing means for drivers as disclosed in U.S. Pat. No. 4,945,334.
In addition to the necessity of maximizing the volume of the tones emitted from the sirens, it is also increasingly desirable to control the directionality of such tones especially in areas where only a portion of the population needs to be warned of impending danger. In conventional apparatuses, multiple independent sets of sirens are required to implement a warning system that can provide anywhere from highly directional to omni-directional emission of warning tones. For example, in a situation where there exists one quadrant of the warning area that need not receive the warning tones, the installer must typically nonetheless purchase an apparatus that is omni-directional and emits an omni-directional warning tone or purchase an apparatus that was intended for omni-directional use but is partially disabled or left incomplete at the factory to accomplish the desired directionality. In each case, the installer typically pays for structure, such as an omni-directional resonance chamber, that is not needed or used.
Improved directionality of warning tones is typically addressed in conventional warning apparatuses by taking a conventional resonance chamber such as disclosed in U.S. Pat. No. 4,796,009 discussed above, and adding horns in the desired direction of the tonal transmission. As discussed above, this approach is wasteful because much of the area of the resonance chamber is left unused, much of the materials used to construct it are wasted, and the cost for this unnecessary structure is passed on to the customer.